In the art of making tires, it may be desirable to employ rubber vulcanizates that demonstrate reduced hysteresis loss, i.e., less loss of mechanical energy to heat. Hysteresis loss can be attributed to polymer free ends within the cross-linked rubber network, as well as the disassociation of filler agglomerates. The degree of dispersion of filler within the vulcanizate can also be important, as increased dispersion may provide better wear resistance.
Functionalized polymers have been employed to reduce hysteresis loss and increase bound rubber. The functional group of the functionalized polymer is believed to reduce the number of polymer free ends via interaction with filler particles. Also, this interaction may reduce filler agglomeration, which may thereby reduce hysteretic losses attributable to the disassociation of filler agglomerates (i.e., Payne effect).
Conjugated diene monomers can be anionically polymerized by using alkyllithium compounds as initiators. Selection of certain alkyllithium compounds can provide a polymer product having functionality at the head of the polymer chain. A functional group can also be attached to the tail end of an anionically-polymerized polymer by terminating a living polymer with a functionalized compound.
For example, trialkyltin chlorides, such as tributyl tin chloride, have been employed to terminate the polymerization of conjugated dienes, as well as the copolymerization of conjugated dienes and vinyl aromatic monomers, to produce polymers having a trialkyltin functionality at the tail of the polymer. Likewise, lithium salts of cyclic amines, such as lithio hexamethyleneimine, have been employed to initiate the polymerization of conjugated diene monomer to produce polymers having cyclic imines at the head of the polymer. These polymers have proven to be technologically useful in the manufacture of tire treads that are characterized by improved traction, low rolling resistance, and improved wear.
While polymers that have functionalities at both their head and tail have demonstrated the ability to provide filler-reinforced vulcanizates with many advantageous properties, the ability to process these polymers can be difficult as compared to non-functional polymers. Namely, the ability to adequately mix filler particles into the rubber compound can be problematic because it can require greater mixing energy and mixing time.
Also, anionically polymerized polymers may have a tendency to flow both during drying (hot flow) and during storage (cold flow). A common solution in the prior art to control this flow is to couple the polymers. For example, these polymers have been coupled with tin tetrachloride or silicon tetrachloride. While these coupling techniques have proven to be technologically useful, further functionalization of the polymers can be difficult.
Because polymers that provide filler-reinforced vulcanizates with reduced hysteresis loss may be important in the manufacture of tires, there is a need to for alternative approaches and improvements.